1. Field of the Invention
The present invention relates to a zeta-potential determining apparatus and more particularly, to a zeta-potential (i.e., electrokinetic potential) determining apparatus that is applicable to monitoring of the performance of a cleaning or rinsing solution in manufacturing of precision industrial products including semiconductor integrated circuits (ICs).
2. Description of the Prior Art
In recent years, the integration scale or level in the technology of precision industrial products, particularly ICs, has been progressing remarkably. To continuously provide the manufacturing processes of these products with higher cleanliness, it is required to enhance the performance of cleaning and rinsing solutions.
Determination of a zeta (.zeta.) potential or electrokinetic potential allows us to know whether the contaminant particles included in a cleaning or rinsing solution for a semiconductor substrate tend to be deposited on the substrate or not. Thus, the zeta-potential determination plays an important role in controlling the performance of cleaning and rinsing solutions in the IC manufacturing technology.
Generally, a particle is not in the electrically neutral state, and it is usually charged positively or negatively. When an external electric field is applied to a liquid electrolyte in which charged particles are dispersed, these particles are moved according to the sign (or, polarity) and magnitude of the charge of the particles in the electrolyte. This phenomena that charged particles contained in a solution are moved by an applied electric field has been known as the "electrophoresis".
If monitor particles, i.e., charged particles for monitoring the mobility, are diffused into a solution and then, the mobility of these monitor particles is measured, a "zeta potential" can be calculated from the measured mobility. The zeta potential is defined as an electric potential generated between the outermost layer of the monitor particle and the solution.
For example, when negatively charged particles of polystyrene latex (PSL) are dispersed in a solution and an electric field is applied across positive and negative electrodes placed apart from one another in the solution, the particles will move toward the positive electrode.
FIG. 1 is a schematic cross-sectional view showing the outline of a cell member of a conventional zeta-potential determining apparatus.
In FIG. 1, a cap 51, which is made of quartz, has a cross section of an inverted U-shape. The cap 51 has a top wall and two opposing side walls. The bottom and the remaining two sides are opened. A bottom plate 52, which is made of quartz, is attached to the opening bottom of the cap 51. Plate-shaped positive and negative electrodes 55 and 56, each of which is made of platinum (Pt), are attached to the two opening sides of the cap 51, respectively. Thus, a cavity 58 of the cell member, which has a shape of a rectangular parallelepiped, is defined by the cap 51, the bottom plate 52, and the positive and negative electrodes 55 and 56.
On measurement or determination, a solution 53, in which negatively-charged monitoring particles 54 such as PSL particles are suspended, is placed or stored in the cavity 58 of the cell member. Then, an appropriate dc voltage is applied across the positive and negative electrodes 55 and 56, thereby moving the monitoring particles 54 in the solution 53, as shown by an arrow in FIG. 1. The mobility of the moving monitor particles 54 is measured in this state.
In recent years, the primary method of measuring the mobility of the monitor particle 54 has been to irradiate a laser beam into the solution 53 from the outside and to determine the mobility utilizing the laser Doppler effect resulting from the motion of the particles 54.
Specifically, as shown in FIG. 2, an incident laser beam L1, which is emitted from a laser oscillator or laser-beam source 101 located outside the cell member, is irradiated to the solution 53 in the cavity 58. The incident laser beam L1 is reflected by the monitor particles 4 to generate a reflected laser beam L2. The reflected laser beam L2 is detected by an optical detector 102 located outside the cell member.
Because the monitor particles 54 are moved in the solution 53 due to the electrophoresis phenomenon, a frequency difference is produced between the incident and reflected laser beams L1 and L2. A data processor 103 performs a frequency analysis using the frequency difference between the two beams L1 and L2, in which a beat frequency is generated by mixing the two frequencies of the beams L1 and L2 and then, the mobility of the monitor particle 54 in the solution 53 is determined from the beat frequency. Further, the data processor 103 calculates a zeta potential based on the determined mobility of the particles 54.
With the conventional zeta-potential determining apparatus shown in FIG. 1, it has been difficult to determine a zeta potential for the solution 53 when the solution 53 has a tendency of generating bubbles 57 in the vicinities of the positive and negative electrodes 55 and 56 due to the applied dc voltage. This is because the moving monitor particles 54 in the solution 53 cannot be distinguished from the bubbles 57 by the laser beams L1 and L2.
In particular, many of the solutions that have been used for cleaning or rinsing a semiconductor substrate in the IC manufacturing technology contain hydrogen peroxide (H.sub.2 O.sub.2). Thus, if an electric field is applied to any one of these solutions, a number of bubbles will be generated in the vicinities of the positive and negative electrodes 55 and 56. Therefore, it is difficult to determine a zeta potential for any of the cleaning solutions, such as a mixed solution (APM) of ammonia (NH.sub.3) and hydrogen peroxide (H.sub.2 O.sub.2), a mixed solution (HPM) of hydrochloric acid (HCl) and hydrogen peroxide, a mixed solution (SPM) of sulfuric acid (H.sub.2 SO.sub.4) and hydrogen peroxide, and a mixed solution (FPM) of hydrofluoric acid (HF) and hydrogen peroxide.